1. Technical Field
The present invention relates to a calibration device capable of calibrating a stereo camera-type range-finding system, a range-finding system including a stereo camera and the calibration device, and a vehicle mounting the range-finding system.
2. Related Art
Collision avoidance systems involving the use of in-vehicle stereo cameras have become more common. A stereoscopic image of the area in front of the vehicle is generated using the stereo cameras, and an obstacle is detected and a distance to the obstacle is measured based on the generated stereoscopic image. The driver can then be alerted to take corrective action to avoid a collision or maintain a safe minimum distance between vehicles. Alternatively, the system can engage a control device such as the brakes and the steering.
The parallax offset may be calculated by memorizing the shape of motionless objects in advance, recognizing a target motionless object by matching a detected target to the memorized shape, calculating a travel distance based on a speed sensor in the vehicle, and correlating the motionless object and the distance change. The parallax offset is then calculated based on the stereo images captured at two different points.
In this example, it is necessary to store the shape of the motionless object (e.g., a green light) in memory to recognize the motionless object from a single luminance image. It is to be noted that, if the memorized object is not present, the control system does not operate. Thus, for example, the system cannot operate on the highway because there are no traffic lights on the highway. Moreover, the shapes and sizes of motionless objects such as traffic signs differ by region and nation, and it is possible that they will be changed in the future. Accordingly, the identification of motionless objects using registered shapes may not be reliable.
Alternatively, feature points may be abstracted from a pattern of shades (shading pattern) on the road ahead of the vehicle, the distance (range) measured using the feature points, road parameters indicating the position and direction of the road calculated, and the parallax offset calculated based on change in the road parameters. As long as the vehicle is moving, a flat road immediately front of the vehicle is likely to be present, and the problem that the target cannot be found is less likely to occur. In this example, recognizing an object of a specific shape is not necessary.
However, in this technique, the information source used to calculate the parallax offset is limited to the road. Consequently, motionless objects other than the road, such as walls, buildings, telephone poles, etc., cannot be leveraged effectively. This limitation becomes a serious problem in use environments other than in vehicles, such as indoors.
In addition, although depending on the camera resolution and correlation point search method, since shading on the road tends to be fine and present few long-range feature points, the above-described technique can only be used to calculate the parallax for close-range areas and cannot be used for calibration. Depending on the purpose, such as automatic adjustment of the distance between vehicles, it is sometimes desirable to set the range-finding range for a vehicle ahead at a farther distance, which may adversely affect the accuracy of the calibration.